LAM - Laboratoire d’Astrophysique de Marseille

Comet Flyby : OSIRIS catches glimpse of Rosetta’s shadow

Publié le 5 mars 2015

Several days after Rosetta’s close flyby of comet 67P/Churyumov-Gerasimenko on 14 February 2015, images taken on this day by OSIRIS, the scientific imaging system on board, have now been downlinked to Earth. With an unprecedented resolution of 11 centimeters per pixel, these data from OSIRIS’ Narrow Angle Camera reveal the most detailed structures on a cometary surface ever. Since at closest approach Sun, spacecraft and comet were almost perfectly aligned, few shadows are visible in the images. As a side-effect of this exceptional observational geometry Rosetta’s shadow on the surface can be seen surrounded by a bright halo-like region.
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The image released today shows an area near the edge of the comet’s belly at the boundary of the Imhotep region covering 228 meters x 228 meters on the comet’s surface. A mesh of steep slopes separates smooth looking terrain from a more craggy area. The image was taken from a distance of six kilometers from the comet’s surface thus making structures with a pixel scale of only 11 centimeters visible. It exceeds all previously taken images of cometary surfaces in resolution.
During the flyby Rosetta not only passed closer by a comet than ever before, but also engaged in a unique observational geometry : For a short time during the maneuver Sun, spacecraft and comet were exactly aligned. “Images taken from this viewpoint are of high scientific value”, says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. Since the surface structures cast almost no shadows, the surface’s reflection properties can b1e discerned. “This kind of view is key for the study of grain sizes”, he adds.
At the top of the image, Rosetta’s shadow can be seen as a fuzzy rectangular-shaped dark spot measuring approximately 30 x 60 square meters. These dimensions are determined by the spacecraft’s penumbra. Such penumbras are especially pronounced when an object is illuminated by more than one light source – or an extended one like the Sun. In both cases light reaches the object from different directions leading to a dark core shadow where the object blocks the entire light source and an adjacent penumbra where only part of the light source is concealed. In this case, the penumbra effect adds about ten to fifteen meters to all sides of Rosetta’s core shadow.
In addition, the region surrounding the shadow appears significantly brighter than the rest of the comet’s surface seen in the image. Scientists refer to this effect as an opposition surge. It is known, for example, from photographs showing astronauts on the Moon and occurs typically on highly structured, regolith surfaces when light incidents from the same direction into which it is reflected. In this situation shadows normally cast by small surface grains disappear leading to a pronounced increase in brightness.
Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta’s Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. Rosetta is the first mission in history to rendezvous with a comet, escort it as it orbits the Sun, and deploy a lander to its surface.
The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d’Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.

Figure 1 : Close-up view of the Imhotep region on comet 67P/Churyumov-Gerasimenko caught by OSIRIS’ Narrow Angle Camera during Rosetta’s flyby on 14 February 2015. Only six kilometers separate Rosetta from the comet’s surface leading to a resolution of 11 centimeters per pixel. At the top of the image Rosetta’s shadow can be seen.